1. Field of Invention
This invention is related to semiconductor integrated circuits and in particular to reducing short circuit current at the output of a CMOS buffer circuit.
2. Description of Related Art
In the buffering of signals there is a need to not only shape the incoming signal but to provide an output that is free from output signal perturbations including the circuit power lines. These perturbations can be caused by a shorting effect in a CMOS buffer output devices of opposite conductivity where one transistor turns on while the other transistor turns off. During the transition both transistors of opposite conductivity, connected to each other and to opposite power lines can provide a path for current to flow from one power line to the other. This shorting current increases power consumption, particularly with drivers that have been bulked up to drive large loads like clock drivers and also produces a noise spike on the power lines. Present practice to reduce this shorting includes trying to control the switching of the output transistors of the CMOS buffer circuit by use of a flip flop like latch connected to the input gate of each output transistor of the CMOS buffer circuit.
In U.S. Pat. No. 4,779,015 (Erdelyi) a low input signal buffer circuit is described where the inverters between the input and the output have fast switching action to reshape the incoming signal into a desired amplitude and pulse width. Referring to U.S. Pat. No. 5,034,623 a low power TTL level CMOS buffer is described where the high and low trip points of the circuitry can be changed by appropriately sizing the transistors. Hysteresis is introduced into the circuitry to control oscillations and to react to a range of TTL signals. In U.S. Pat. No. 5,399,924 (Goetting et al.) is a low power inverter circuit which can be configured to be a conventional CMOS inverter in which the circuit can be switched between an inverting and non-inverting buffer.
In the references, the same basic CMOS inverter was used associated with other logic to accomplish various functions and to minimize power consumption. None of these provided for the protection of shorting at the output transistors which becomes important when the transistors of opposite conductivity in the CMOS inverter are switched from "on" to "off" and "off" to "on" simultaneously.